In a typical manhole several sewer lines converge from different directions and the bottom of the manhole serves to provide a leak proof interconnection between the lines. The bottom of the manhole, however, is exposed to corrosive gases and liquids, which over time cause a deterioration of the bottom mortar sealings. These, therefore, begin to crumble and leak so that repairs need to be made. The repair has involved the insertion of a plastic invert supported by appropriate mortar, which in this case does not get exposed to the corrosives to the same extent. The use of plastic inverts has also been extended to new manhole constructions so as to extend their useful life.
Patents for plastic inverts have been described in the art, see for example U.S. Pat. Nos. 4,483,643 to Guggemos and 3,403,703 to Reimann. Other U.S. patents describing various ways for interconnecting sewerlines at the bottom of a manhole are U.S. Pat. Nos. 4,253,282 to Swartz, 3,7878,078 to Williams and 4,341,236 to LaBenz.
In these prior art inverts little room is available to adjust a preformed plastic invert to differently sized manholes while also accommodating sewer line connections from different directions. For example, the Reimann patent describes a preformed access channel of a fixed length which in effect requires building of the manhole around the invert to be placed at the bottom.